To those who do, here's some questions, feel free to describe your own creations!

What inspires your costumes? Do you base them on a series or fandom, or are they completely your own creation?

Do you give your characters a backstory, or come up with "in-character" explanations for parts of the costume?

Overall, why do you do it? Do you like the freedom of original characters/designs? Do you like the challenge of coming up with a design from scratch, or do you find it more personal than strict recreation?

I'll start this off with my own answers:

1- Inspiration.
Lately a lot of my planned cosplays are personifications of some of NYC's skyscrapers which I am a fan of. I've been a huge skyscraper fan ever since I was a kid, drawing them in personified/avatar form for about as long as I can remember. When it comes to designing clothing for these skyscraper-folk to wear, I draw inspiration from lots of real-world sources. Sometimes I feel a bit like the costume designers from Star Wars, who used a ton of real world historical and ethnic influences, blending them together to create amazing looks for their characters, a whole visual language of sorts. Bet you didn't know that Amidala's episode 1 Senate Gown was based very much on a Mongolian women's outfit (even that hairdo!), or that Anakin's civilian disguise in episode two used real African mudcloth for its vest!
For my Skyscraper-folk, the actual buildings they're based on give me lots to work with too. Whether it's the bold vertical and horizontal elements of 55 Water St's facade, or the countless art deco details large and small of the Empire State or Chrysler, it's fun figuring out ways to weave those into an outfit that looks like something a living being would wear as either everyday, ceremonial, or battle dress.

2- backstory
Heck yes, I give my characters a backstory! Not just how the heck a skyscraper gains an Avatar in the first place, or the culture that these beings might create for themselves, but the history of an individual building figures in there as well. For example,One New York Plaza is a survivor of multiple fires, so she wears phoenix heraldry. One Penn Plaza may be a modernist skyscraper, but he wears some art deco elements, as a tribute to his elders.

3- why?
Short answer- it's fun! I get to combine two of my passions, cosplay and architecture. And in doing so, maybe causing people to look at something familiar, and see it in a new way. Recently, somebody online looked at the character drawings in my deviantart gallery and said (after googling buildings they were not familiar with) how well I had captured said building's nuances.
I love taking the ideas that have been in my head for years and bringing them to life though art, writing, and yes, cosplay!

I do! I'll have to edit this with more detail later on, but a lot of my inspiration comes from small tings I happen to come across. For example, I got an idea for a Medusa costume from the watch/bracelet my mom got me for Christmas. I figured, 'hey, why not do a snake lady costume? Medusa! I can make her dress like this to represent this and have her hair like this with this sewn in and raaawwwwrrrr!!!'

Inspiration:
I usually come up with my OC concepts through roleplay. I play a lot of D&D and I also LARP, so I am constantly creating new characters and some of them I just love so much! With LARP I usually end up putting costumes together for them anyway because that is part of the experience, so I may as well show them off!

Backstory:
Because my characters usually come from roleplay, they usually have extensive backstory. I spend a lot of time during character creation coming up with a history for my characters, and I usually have even the smallest details figured out. I have an entire folder on my computer for word docs containing the histories of my characters xD I think it's super important to have history for your character, because it gives you more insight into how they act, think, talk, etc.

Why?
Why not? I don't often wear my OC cosplays to conventions, but I do love doing photoshoots and meetups with them. Being a fictional character is fun on its own, but being a character I created is awesome because I know that character's personality so well and can easily be IN character. Long story short, it's just plain fun. It's like playing pretend. lol

Oooooh yes. The only one I've been putting effort into fleshing out is Candie Keane the Zombie Chick.. I have a million charaters, but she's the only one I came up with specifically for cosplay.

-Inspiration- To be completely honest, I wanted an interesting costume for the bar we didn't go to on Halloween this year that people would recognize as a "traditional" Halloween costume. I knew I wanted to wear a specific dress, I knew I wanted to do makeup and I new I wanted to wear Snow's curly pink wig. I ran from there. I usually take a general idea and go on a dollmaker site and use ALL the makers and see what I can come up with, what is new and what works better. She started out entirely fluorescent green, with curly cotton candy pink hair in pigtails with a giant lolipop and a black strapless dress. She's evolved into a more realistic zombie color, with actual stitches in realistic places and she wears a raspberry pink wig instead of having magic milkshake pink hair. She still wears the black dress, but she's also a lounge singer, so why the hell not?

-Backstory- After we didn't go to the bar that night, I decided she needed a place to live and friends to play with and a complete and total overhaul. Then I decided I was going to make a comic, "Finn's Comic; starring Candie Keane and Friends." I get really way too into giving characters backstories. My favorite is "this is this way because of why?" I'm still working on a plausible reason for why she's not mindless.. I may have to jam her into the Ugly Americans fandom to solve that issue and I really would rather not do that.

-Why- Characters just kind of pop into my brain and go "HERE I AM. HERE'S MY FACE, NOW DO SOMETHING WITH ME." And I pick them up, take them in and give them homes. If I don't put them on paper, they sit in my head and scream until I do. ....and that's all I have to say about that.

1: they start out based on another series/fandom, or even just things I see in real life or on TV, but slowly start changing and becoming more and more original that you can no longer tell.
2: well, my first OC started to get a little 'lonely' so I started making a backstory then making more characters and now there's a whole story around them that hopefully one day I will be able to finally make the comic ;A;
3: the freedom, I just love making things the way I want them to be. they all come out exactly how I want them, not how anybody else does. They're mine to call my own and say, "hey! I made this character up myself."

I love doing original character cosplay. The first group of cosplay I ever made to attend A-kon in 2004 was all original characters for the six of us. They had stories because they were from a book my friend and I were working on at the time.

As for answers to your questions:
Inspiration: I get inspired by so many things. Right now I'm working on a steampunk wind-up doll inspired by Abney Park's song "Herr Drosselmeyer's Doll" and a yuki-onna inspired by Japanese Folklore to fit with A-kon's theme for this year (we will have a whole group of yokai for the theme, each will be designed and made by me, mixing traditional Japanese looks with my own twists to them). I sometimes see a piece of fabric or jewelry and start thinking about what I can create with them. Other times I see a type of character or thing and want to make a costume for it. I've started working on costumes for the four seasons, Fall being complete, and have often thought of doing costumes based on flowers (even though I'm so far from girlie most of the time) because you can do something beautiful with them. You just never know where inspiration can come from.

Back-story: Yes, some characters do have deep backstories. Others have just little bits and pieces here and there. Again, for my wind-up doll, I already have explanations for why she will have certain things or why stuff is going on with the outfit. It's just how my mind has developed the costume. Others might not get a lot of explanation, but they don't go without reason for things.

Why: Like others have said, why not? But I love the creative process. And as much as I love to make things based on already established series or art, I love to create my own things. I've been designing clothing for the last fifteen years and always like to bring my own designs to life along with the ones I do based on anime/manga/video games/movies...whatever. It's the thought of being able to find what I want to use and use it without anyone second guessing my choice. Creative, artist license. That's what I love about originals.

1)I love getting inspired by fairy tales and other stories that are often re-envisioned...Though to be honest your inspiration has me wanting to do a cosplay based around the Russian Orthodox Church <3
The only one I have done so far is the March Hare.
2)For my March Hare I have watched a ton of movies and read many different versions of this character but most the time it is still vague. So I decided to add the character influences around the March Hare along with the scenery. I basically take the same ideas for my other OCs that I shall cosplay eventually~
3)I like the freedom of finding something I like and going, yeah this character would totally have this with out having to look up references. Also it kind of cracks me up when people try to name the series you are from and you just have to keep telling them no XD

1.) I've spent a lot of time around costumers who never do "cosplay" (as in dressing up as existing characters). That's the sort of creativity that got me into going to cons in the first place even though I currently do a lot of what they call "re-creations".
History is my biggest inspiration. I'm a bit of a research nut. Honestly, I don't think I'm terribly creative at costume design, though. A lot of my projects revolve around an existing character or idea with an historical bent (like DDR Kabuki).

2.) Not really. Sure, I've done some costumes that are based on another creation of mine (the Red Rebel, which isn't in my gallery at all, for example, is from a comic I drew when I was in high school... and was made in high school). Sure, I've done some like the OP did where you take an idea (aka a DDR game) and turn it into a costume. But do I go and create characters with backstories specifically to cosplay? Not often. The only one I can think of that's not from a fanfic or comic I wrote is the Twelfth Doctor, and that's already based on an existing universe. I didn't plan to create a "backstory", per se, but as I was designing the costume, I thought, What is her personality like? What traits did she pull from past regenerations? which became, How did she regenerate from Smith's Doctor in the first place? Why did she end up female? Who was her companion(s) at the time? And it grew into a story in my head.

3.) Because why not? Costuming hasn't always been about recreating characters. That's relatively new. Costumes are an art form and can be whatever we want them to be. I love love love love love being characters, but sometimes I want to design something new to wear.

Inspiration -- well, I don't do a lot of original characters, but I do a lot of original designs--like, a character from a book, where there's maybe a description of the character but not an illustration. So I guess the books that I do costumes from would be my inspiration. Sometimes, like with my recent Mara Jade costume, I'll start with a cover illustration as inspiration but then turn it into something original (since authors almost never have any say in cover illustrations, I don't take those as canon depictions of characters). I usually try to adhere to the book's description but sometimes I'll ignore pieces if I think something else would work better, same as someone might do if they were making a movie of the book.

Backstory -- well, with book characters, they have plenty of backstory.

Why? I read a lot, probably more than I watch tv or movies or read comics, and I find myself attached to the characters, same as I would in a visual medium. Plus well, I like the challenge of trying to create something where anyone who's read the book would recognize me, even if their own mental image of the character had been totally different. I think doing my own designs is more fun that doing a strict reproduction of something, and doing cosplays from books lets me design stuff while at the same time enjoying the experience of dressing up as a character I love.

My love for fashion design and anime inspires my creations and I'm hoping to cosplay my 1st Oc for the 1st time either at Anime USA or Halloween. I based most of them off my orignial stories, but the one I'm cosplaying soon is going to be from Young Justice/ DC comics

Everyone of them have backstories, some more in depth than others. I create OC's for the freedom and flexibility. I feel proud and accomplished when I see the finished creation.

I'm planning my fist cosplay of an OC. I am very excited to be able to do this, though I'm not sure how to do a lot of it, so it may be a while before it's started.

1- Inspiration.
My OCs spawn from RPGs more often than not. The character I'm going to cosplay is an OC from a Pokemon RPg I was a part of for a long time. She's a Torchic Pokemorph (see: Gijinka) and her hair will be incredibly complicated, but I'm still very excited.

I had several characters on the RPG, with 3 that I used very often. They are very close to my heart. <3 All of my OCs that I give backstories and developed prsonalities have a special place in my heart. XD

2- Backstory
Yes. Every character I imagine has a very detailed backstory. Some are more intricate than others, but they all have a story. My most complicated character is probably... any character in the "Sato" family. They are a family of characters I thought u a few years ago. It's hard to explain so I won't go into detail, but to make all of their backstories intertwine and make sense, I spent months writing up timelines, giving specific dates for certain events in their life so everything has a chronological order, and so on. Lots of detail. XD

3- Why?
Because it's fun. I don't "have" to do any of this by any stretch of the word. I do it because it brings me a special kind of joy to see my characters come to life with all the detail I get them. I want to cosplay more of my OCs as I get more money to fundmy cosplaying habits.

Inspiration- all of my OCs (only one of which is listed on this site) are characters from various tidbits or stories that I have been or am writing at this time. I set up their costumes so I could get an idea of how they'd look in real life (strange, I know, but it helps me get my thoughts together).

Backstory- they're characters from stories/books, so of course they have backstories... and extensive ones, at that. Not a single character of any of my books goes through the creative process without having an extensive backstory.

Why- I honestly don't know, but I love it.

__________________"Let's put a cookie on a plate and see what happens to the poor bastard who tries to steal it!"
-Sword Dude to Kari during a pointless argument

I've done an original character based on an existing design and universe, for entirely practical reasons. I knew that if I dressed as Neytiri or Grace from Avatar I would drive myself crazy trying to recreate their exact stripe patterns. That didn't sound like fun, so instead I invented another AVTR driver scientist and yes, gave her a name and a bit of backstory. I still call it a recreation costume, though.
I've got nothing but respect for people who design original costumes from scratch. My brain just doesn't do that.

What inspires your costumes? Do you base them on a series or fandom, or are they completely your own creation?
I've always liked personifying things - places, structures, landforms, holidays, etc. I've drawn and written characters like this for years. I think there's a lot of "character" in things and concepts we interact with daily, and it's fun to make an actual character out of that. As an extension, it's fun to costume that character, because it's another way to realize the design, it's fun to explain it to people and get them to think about the character of things, and it's fun to think about how to roleplay something that's not entirely human.

Do you give your characters a backstory, or come up with "in-character" explanations for parts of the costume?
Because my characters are based on real things, they come with a backstory rooted in actual history. That's a particularly concrete aspect for things that exist on a human timescale (structures, days, etc), but more abstract and interpretable for things like geologic features.
As for explaining features of the costume, I base them heavily on features of the thing I'm personifying. It has to be recognizable, after all! If the thing is architectural, there's a lot to go off of, design-wise. If it's geological, I tend to base a lot of things on rock types. If it's more conceptual, then yes, all the aspects tend to be related to the history.
(And yes, I've cosplayed as a day of the year, a structure, and an earthquake fault in the past.)

Overall, why do you do it? Do you like the freedom of original characters/designs? Do you like the challenge of coming up with a design from scratch, or do you find it more personal than strict recreation?
It's just fun? I like coming up with the characters, and writing them and drawing them, so why not cosplaying them?
Also, in the case of the San Francisco landmarks I've been cosplaying lately, it's a bit of a way to feel closer to the city even though I can't live there yet.

What inspires your costumes? Do you base them on a series or fandom, or are they completely your own creation?
I generally look to my favorite characters from other fandoms to take cues and ideas from when I am in the design process . I also look to my own wardrobe when it comes to designing clothes.

Do you give your characters a backstory, or come up with "in-character" explanations for parts of the costume?
Um. Yes. Character backstory should be one of the first things to get done!

Overall, why do you do it? Do you like the freedom of original characters/designs? Do you like the challenge of coming up with a design from scratch, or do you find it more personal than strict recreation?
It's definitely more personal for me. This is a character I've had for over 2 years and she's also my most beloved and well known. I want her to get more recognition! Raine-chan loves attention.